Atrocity stories are nothing else than lies; the two words "atrocity stories" simply pretend to be more diplomatic.
The purpose is to destroy an image of the enemy, to create a new one, mostly a bad one. The story creating the image is not necessarily made up completely. It can also be a changed into a certain variable direction.
The most important thing about atrocity stories is to follow the line of possibility. Even if the whole story is made up it must be probable or at least possible, following rumors. Most successful might it be if a rumor is spread on purpose, some time before the atrocity story is launched, because as soon as something seems to be familiar, it is easier to believe it.
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The water-clocks are an early long-distance-communication-system. Every communicating party had exactly the same jar, with a same-size-hole that was closed and the same amount of water in it. In the jar was a stick with different messages written on. When one party wanted to tell something to the other it made a fire-sign. When the other answered, both of them opened the hole at the same time. And with the help of another fire-sign closed it again at the same time, too. In the end the water covered the stick until the point of the wanted message.